1. Field of Invention
The invention relates to a film die for the production of tubular film made of thermoplastic synthetics. The film die has at least one feed channel for synthetic fused material which ends in a ring-shaped discharge nozzle with an interior and/or exterior cooling ring that is adjusted to the diameter of the extruded synthetic tube.
2. Description of the Prior Art
Film dies of this type are known in various embodiments, whereby the interior cooling air supplied to and discharged through the film die in axial direction serves for the cooling of the extruded film tube and for the inflation of the film tube into a tubular film bubble, which stretches the fusible synthetic tube. However, film dies exist, in which no air is supplied and/or discharged in the film bubble during the extrusion process, but instead a predetermined static air volume is present in the film bubble.
Generally, exterior cooling rings have a ring-shaped nozzle gap that is directed toward the extruded film tube, through which cool air is blown so that the same is rapidly cooled down in order to improve the output with a so-called frost pipe that is as low as possible.
In order to control the thickness of the film tube blown into a tubular film bubble across its circumference, it is known to variably heat and/or cool the tube-shaped extruded synthetic fused material section by section in order to influence the thickness profile. For this purpose the effect is being utilized that while inflating the synthetic tube into a tubular bubble, the warmer areas are stretch formed at a stronger degree, and the cooler areas at a lesser degree. For this purpose, physical parameters of the cool air are varied sector by sector. These parameters include the temperature and the volume flow of the cool air.
It is known from DE 42 07 439 that the exterior cooling ring is divided into sectors, in which the air blown onto the extruded film tube can be heated and/or cooled. This film die has a compact construction, because the elements for the variable heating or cooling of the air blown onto the extruded film tube are directly integrated into the exterior cooling ring. Heating cartridges distributed across the circumference may be arranged on it for the heating, and Peltier elements distributed across the circumference may be arranged on it for the cooling. The variation of the physical parameters of the cool air sector by sector can also occur on or through the interior cooling ring in addition, or as an alternative.
On the other hand, film dies have been known from DE 42 36 443 A, in which heating cartridges distributed across the circumference are integrated into the film die itself, whereby these heating cartridges each can be activated in order to control the thickness of the film tube. A convertibility of the temperature of the die section by section results from this construction. However, this system has the advantage of a more lethargic control as opposed to the system with the exterior cooling ring.
Even though the film die known from DE 42 07 439 A enables a quicker and more exact control of the cool air escaping from the film die and blown onto the film tube, inaccuracies with the control of the wall thickness of the blown film tube still exist, in that interactions between the air of variable temperatures flowing from the exterior cooling ring and the ambient air occur within the area of the nozzle discharge of the exterior cooling ring. Depending on changes in ambient conditions, as the case may be, unsteady turbulences are created here, which influence the cooling results in the area of the film tube in an unpredictable manner.